In the manufacture of semiconductor devices, photolithography is often used, especially in view of the circuit patterns of semiconductors being increasingly miniaturized in recent years. Projection optics are used to image a mask or reticle onto a wafer and as circuit patterns have become increasingly smaller, there is an increased demand for higher resolving power in exposure apparatuses that print these patterns. To satisfy this demand, the wavelength of the light source must be made shorter and the NA (numerical aperture) of the optical system (i.e., the projection lens) must be made larger.
Optical systems having a refractive group have achieved satisfactory resolutions operating with illumination sources having wavelengths of 248 or 193 nanometers. At these wavelengths, no or only slight correction of chromatic aberration is needed. As the element or feature size of semiconductor devices becomes smaller, the need for optical projection systems capable of providing enhanced resolution increases. In order to decrease the feature size which the optical projection systems used in photolithography can resolve, shorter wavelengths of electromagnetic radiation must be used to project the image of a reticle or mask onto a photosensitive substrate, such as a semiconductor wafer.
Because very few refractive optical materials are able to transmit significant electromagnetic radiation below a wavelength of 193 nanometers, it is necessary to reduce to a minimum or eliminate refractive elements in optical projection systems operating at wavelengths below 193 nanometers. To date, no second optical material is known which allows for chromatic aberration correction at wavelengths below 160 nm or shorter with sufficient material properties (homogeneity property, availability). Consequently, one has to construct a catadioptric imaging system, such as the present one, in order to allow for correction of chromatic aberrations with the use of only one single material, especially, SiO2 or CaF2.
The desire to resolve ever smaller features makes necessary optical projection systems that operate at the extreme ultraviolet wavelengths, below 200 nm; and therefore, as optical lithography extends into shorter wavelengths (e.g., deep ultraviolet (DUV) or very ultraviolet (VUV)), the requirements of the projection system become more difficult to satisfy. For example, at a wavelength of 157 nm, access to 65 nm design rules requires a projection system with a numerical aperture of at least 0.80. As optical lithography is extended to 157 nm, issues relating to resist, sources and more importantly calcium fluoride have caused substantial delays to the development of lithography tools that can perform satisfactorily at such wavelengths. In response to the technical issues relating to the source and the material, it is important that projection system development investigates and focuses on maximizing spectral bandwidth to an order of 1 pm, while simultaneously minimizing the deficiencies associated with the materials that are used, i.e., it is desirable to minimize the calcium fluoride blank mass.
It has long been realized that catadioptric reduction optical systems (i.e., ones that combine a reflective system with a refractive system) have several advantages, especially in a step and scan configuration, and that catadioptric systems are particularly well-suited to satisfy the aforementioned objectives. A number of parties have developed or proposed development of systems for wavelengths below 365 nm. One catadioptric system concept relates to a Dyson-type arrangement used in conjunction with a beam splitter to provide ray clearance and unfold the path to provide for parallel scanning (e.g., U.S. Pat. Nos. 5,537,260; 5,742,436; and 5,805,357). However, these systems have a serious drawback since the size of the beam-splitting element becomes quite large as the numerical aperture is increased, thereby making the procurement of optical material with sufficient quality (in three dimensions) to make the cube beam splitter a high risk endeavor, especially at a wavelength of 157 nm.
The difficulties associated with the cube beam splitter size are better managed by locating the cube beam splitter in the short conjugate of the system, preferably near the reticle or at its 1× conjugate if the design permits. Without too much effort, this beam splitter location shrinks the linear dimension of the cube by up to 50%, depending upon several factors. The advantages of this type of beam splitter placement are described in U.S. Pat. No. 5,052,763 to Wilczynski. Further, U.S. Pat. No. 5,808,805 to Takahashi provides some different embodiments; however, the basic concept is the same as in Wilczynski.
It is also possible to remove the cube beam splitter entirely from the catadioptric system. In one approach, an off-axis design is provided using a group with a numerical aperture of 0.70 operating at 248 nm. In U.S. Pat. Nos. 6,195,213 and 6,362,926 to Omura et al. disclose other examples of this approach and U.S. Pat. No. 5,835,275 to Takahashi illustrates yet another example. Takahashi et al. offer several similar examples of beam splitter free designs in European patent application EP 1168028.
Most of these “cubeless” embodiments share a common theme, namely that the catadioptric group contains only a single mirror. Additional mirrors can possible be used to improve performance. However, pure reflective designs with multiple mirrors have been investigated but have largely failed because these designs have proven unable to achieve adequately high numerical apertures (e.g., U.S. Pat. Nos. 4,685,777; 5,323,263; 5,515,207; and 5,815,310).
Another proposed solution is disclosed in U.S. Pat. No. 4,469,414 in which a restrictive off-axis field optical system is disclosed. The system disclosed in this reference does not include a doubly passed negative lens in a first partial objective. Further, the embodiments disclosed therein are of impractical geometry and of far too low numerical aperture to provide improved lithography performance in the ultraviolet wavelength region.
In conventional practice, four mirror catadioptric configurations typically are limited in terms of their numerical apertures due to the location of the refractive lens part relative to the mirrors of the system.
U.S. patent application publication No. 2002/0024741 discloses various projection optical systems including one in which a lens element is positioned spatially in front of mirror M3; however, in this embodiment, the lens element that is positioned in front of mirror M3 is a double pass type lens element. The use of a double pass lens element complicates the system design because the use of a double pass lens between mirrors M3 and M4 requires the double pass lens to be close to mirror M4 and therefore it is difficult to mount.
In addition, there are a number of other differences between the system disclosed in this published application and the present system. For example, FIG. 26 of the 2002/0024741 publication discloses a single pass lens element optically disposed between the very first mirror and the very last mirror of the whole system. Unfortunately, this element is very large in diameter and therefore difficult to manufacture. The disadvantage of such a single pass lens element is that it either requires a lateral separation of beam bundles traveling between the various mirrors, resulting in a large diameter of the lens or that it has to be physically disposed between the backside of mirror #1 and the backside of mirror #4 as shown in each of the FIGS. 23 to 28, again leading to a large diameter of the lens. As will be described in greater detail hereinafter, the present embodiments do not suffer from this disadvantage since the present lens elements are not required to be very large in diameter. With respect to the location of the aperture stop, the embodiments in the publication (as shown in FIGS. 20–28) have an aperture stop located in front of the refractive group Gr2 with the aperture stop separating it from the field mirror group Grf.
U.S. Pat. No. 5,323,263 to Schoenmakers discloses an embodiment in which there are multiple mirrors used in which a number of lens elements are disposed between the most optically forward mirror and the second most optically forward mirror. The lens elements between these two mirrors are all single mirrors.
What has heretofore not been available is a catadioptric projection system, especially a four mirror design, that has particular utility in 157 nm lithography and produces an image with a numerical aperture of at least 0.80 and includes other desirable performance characteristics.